Production method for fluid dynamic pressure sintered bearing

ABSTRACT

A production method for a fluid dynamic pressure sintered bearing includes: preparing a sintered bearing having a porosity of 8 to 20 vol % as a material; and controlling at least one of an overall length, an outer diameter, and an inner diameter of the sintered bearing by repressing the sintered bearing. The production method further includes: forming grooves for generating a fluid dynamic pressure on a bearing surface of the sintered bearing by performing repressing and plastic working on the sintered bearing; and sealing pores exposed on the bearing surface by infiltrating a resin into at least the pores; and barreling entire surface of the sintered bearing by magnetic barreling or electromagnetic barreling.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a production method for a fluid dynamicpressure bearing composed of a sintered compact. In particular, thepresent invention relates to a production method for a fluid dynamicpressure sintered bearing which is desirably used for compact drivingmotors of various information devices, for example, the driving motorsbeing driving sources of disc drive devices (which read and writeinformation from and to a magnetic disc or an optical disc such a CD ora DVD) or polygon motors of laser printers.

2. Description of Related Art

In these kinds of compact driving motors of various information devices,not only rotational performances of high speed and high precision butalso mass production, low cost, and low noise are required to beimproved. Whether these properties are good or not depends on a bearingwhich supports a shaft. In recent years, the above fluid dynamicpressure sintered bearing has been widely used as a bearing which canmeet the above requirements. In the fluid dynamic pressure sinteredbearing composed of a sintered compact, an oil film composed oflubricating oil is formed in a small gap between a shaft and thebearing, and the oil film is compressed by rotating the shaft, so thatthe shaft is supported with high stiffness. This fluid dynamic pressuresintered bearing is known as a non-contact type bearing. Grooves forgenerating a fluid dynamic pressure are formed on bearing surfaces (aninner peripheral surface and an end surface) on which the shaft slides,so that the generated fluid dynamic pressure can be effectivelyobtained. The grooves may be herringbone grooves or spiral grooves (seeJapanese Unexamined Patent Application Publication No. 2003-262217).

The fluid dynamic pressure sintered bearing has pores. For example, afluid dynamic pressure sintered bearing for compact motors has aporosity of about 15 vol %. When the pores exist on a bearing surface,lubricating oil infiltrates the pores of the bearing, the pressure ofoil film is decreased, so that effects of fluid dynamic pressure aredecreased. In order to solve this problem, the sealing of at least thepores on the bearing surface is desirable and the decrease of fluiddynamic pressure is thereby prevented. The sealing uses mechanicalimpacting (for example, shot blasting or sand blasting) or infiltrationof suitable resin into pores (see Japanese Unexamined Patent ApplicationPublication No. Hei 11-62948).

However, in the sealing by the mechanical impacting, grooves formed forgenerating a fluid dynamic pressure may wear, an inner peripheralsurface shape of the bearing may be deformed, and it is difficult tosufficiently seal the pores. Due to these, the fluid dynamic pressure isinevitably decreased to some degree. On the other hand, when the resininfiltrates the pores, the sealing condition is good. However, it isdifficult to sufficiently remove the resin on the surface by waterwashing, and the resin inevitably remains thereon. Due to this, sizeprecision of the bearing is deteriorated. When plastic working byrepressing (sizing) after the resin infiltration is performed so thatgrooves for generating a fluid dynamic pressure are formed, the resin isadhered to a male die of sizing core or the like. In the case ofrepressing by using the male die to which the resin adheres, sizeprecision of the bearing is deteriorated and deformation thereof occurs.Since the fluid dynamic pressure sintered bearing is porous, even whenejection of green compact, which corresponds to the fluid dynamicpressure sintered bearing, from die is performed after compacting in thedie, a spring back is small and transfer properties of grooves are good.However, when the fluid dynamic pressure sintered bearing which has theinfiltrated resin is repressed, a spring back becomes large due todecrease of pores, and the amount of the spring back is uneven.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide a productionmethod for a fluid dynamic pressure sintered bearing, which can preventchange of the bearing in size after repressing and thereby can improvesize precision of the bearing, thereby improving rotational performancesof high speed and high precision and bearing properties of low noise.

According to one aspect of the present invention, a production methodfor a fluid dynamic pressure sintered bearing includes: preparing asintered bearing having a porosity of 8 to 20 vol % as a material; andcontrolling at least one of an overall length, an outer diameter, and aninner diameter of the sintered bearing by repressing the sinteredbearing. The production method further includes: forming grooves forgenerating a fluid dynamic pressure on a bearing surface of the sinteredbearing by performing repressing and plastic working on the sinteredbearing; and sealing pores exposed on the bearing surface byinfiltrating a resin into at least the pores; and barreling entiresurface of the sintered bearing by magnetic barreling or electromagneticbarreling.

In the production method of the present invention, the fluid dynamicpressure sintered bearing is obtained as follows. That is, both the sizecontrol of the whole sintered bearing and the formation of the grooveson the bearing surface are performed by the repressing. Next, the poresexposed on the bearing surface are sealed by the resin infiltration.Finally, the sintered bearing is subjected to the magnetic barreling orthe electromagnetic barreling. Thus, the fluid dynamic pressure sinteredbearing is obtained in the above process order. In this productionmethod, since the sealing of the pores is first performed by the resininfiltration, in comparison with a case sealing is performed bymechanical impacting (for example, shot blasting), deformation of thegrooves, the inner peripheral surface supporting the shaft, and the likecan be prevented, and decrease in the fluid dynamic pressure can beprevented by the sufficiently sealing of the pores.

When the pores are sealed by the resin infiltration in the conventionaltechnique, the resin remaining on the surface of the sintered bearingafter water washing of resin causes decrease in size precision. Incontrast, in the production method of the present invention, since thesintered bearing is subjected to the barreling after the resininfiltration and the entire surface of the sintered bearing is polished,the resin remaining on the surface is removed by the barreling, so thatsize precision of the sintered bearing can be secured. Since the resininfiltration is performed after the all repressing, adhesion of theresin to the male die for the repressing can be prevented, so thatdeterioration of size precision, which may be caused by the adhesion,can be prevented. Since in the repressing, the sintered bearing issimply composed of sintered compact having the unsealed pores, thespring back after the repressing of the sintered bearing and theejection of the sintered bearing from the die is maintained to be smalland the transfer properties of the fluid dynamic pressure grooves ontothe sintered bearing are maintained to be good.

The barreling of the present invention is limited to magnetic barrelingor electromagnetic barreling. In these barreling, in a barrel (vessel)having a spatial magnetic field generated therein, works are agitatedtogether with plural fine media, and the media give impacts to theworks, so that a fine burr and irregularity existing on surfaces of theworks are removed and the surfaces of the works are thereby flat andsmooth. In particular, these barreling are typical methods which aredesirably used for final finishing of works having complex shapes. Inparticular, when these barreling are used for the bearing of the presentinvention, these barreling can give impact effects of the media to theinner peripheral surface of the bearing without damaging the shapes ofthe fluid dynamic pressure grooves formed on the bearing. In theproduction method of the present invention, by the barreling finally onthe bearing, the resin, which remains on the surface after the cleaningof the bearing in the above manner, is removed, so that the bearingsurface becomes clean. In addiction, the remaining pores are closed byplastic flow due to the impacts of media, thereby being completelysealed.

Since the entire surface of the sintered bearing, which includes theinner peripheral surface, is cleaned by the barreling, for example,resin coating for improving the sealing effects can be desirablyperformed. For example, a resin coating layer can be formed on theentire surface which includes the inner peripheral surface. The resincoating layer can be composed of a fluororesin and have a thickness of 5μm or less. Since the entire surface is oil-repellent to the lubricatingoil by the resin coating, even when the pores remain inside the bearing,the infiltration of the lubricating oil into the bearing can beprevented, so that fluid dynamic pressure effects can be more improved.

In the production method of the present invention, both the size controlof the whole sintered bearing and the formation of the fluid dynamicpressure grooves on the bearing surface are performed by the repressing.Next, the pores are sealed by the resin infiltration, and the sinteredbearing is subjected to the magnetic barreling or the electromagneticbarreling. By performing the processing in the above process order, thepores can be sufficiently sealed and the decrease of fluid dynamicpressure can be prevented effectively. In addition, since the sizeprecision can be improved, bearing performances (for example, rotationalperformance of high speed and high precision, and low noise) can beimproved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal cross sectional view showing a fluid dynamicpressure sintered bearing produced by a production method of anembodiment according to the present invention.

FIG. 2 is a plan view showing the fluid dynamic pressure bearing shownin FIG. 1.

FIG. 3 is a cross sectional view showing the fluid dynamic pressurebearing shown in FIG. 1.

FIG. 4 is a diagram showing a process order of the production method ofthe embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention will be described hereinafterwith reference to the drawings.

FIG. 1 is a longitudinal cross sectional view showing a fluid dynamicpressure sintered bearing 1 (hereinafter simply referred to “bearing”).The bearing 1 is cylindrical and is produced by a production method ofthe embodiment according to the present invention. FIG. 2 is a plan viewshowing the bearing 1 shown in FIG. 1. FIG. 3 is a cross sectional viewshowing the bearing 1 shown in FIG. 1. The bearing 1 is a compactbearing which is desirably used for spindle motors of magnetic recorddisc drive devices. For example, the bearing 1 has an outer diameter ofabout 5 to 6 mm and an inner diameter (that is, a diameter of shaft hole11) of about 2 to 3 mm. The bearing 1 is composed of a sintered compactobtained by sintering a green compact formed by compacting a raw metalpowder. The bearing 1 has a porosity of 8 to 20 vol %. In practical useof the bearing 1, lubricating oil infiltrates pores of the bearing, sothat an oil-impregnated sintered bearing is obtained.

The bearing 1 rotatably supports a shaft (denoted by reference numeral 2shown in FIG. 3) which is inserted into the shaft hole 11 of the bearing1. In this case, the shaft 2 has a shaft body, which is inserted intothe shaft hole 11, and a flange which is formed on the shaft body. InFIG. 1, the shaft body 2 is inserted into the shaft hole 11 from theupside, and the flange faces an upper surface of the bearing 1. A radialload of the shaft 2 is supported by an inner peripheral surface 13 ofthe bearing 1, and a thrust load of the shaft 2 is supported by an uppersurface 12 of the bearing 1.

As shown in FIG. 2, on the upper surface 12 of the bearing 1, pluralspiral grooves 14 are formed at equal intervals in one circumferentialdirection. The spiral grooves 14 extend so as to inwardly curve toward arotation direction R of the shaft 2. In FIG. 2, the spiral grooves 14are shown by using hatched lines so as to be distinguished from theupper surface 12. End portions on peripheral sides of the spiral grooves14 open to a peripheral surface. End portions on inner peripheral sidesof the spiral grooves 14 do not open to an inner peripheral surface 13of the shaft hole 11 so as to close. The number of the spiral grooves 14is about 10 (for example, 12 in FIG. 2). The maximum depth of the spiralgroove 14 is about 10 to 20 μm.

As shown in FIG. 3, plural separation grooves 15 are formed at equalintervals in a circumferential direction on the inner peripheral surface13 of the shaft hole 11 of the bearing 1. The separation grooves 15 aresemi-circular arcs in cross section, and straightly extend from one endsurface of the bearing 1 to the other end surface thereof in an axialdirection. Eccentric grooves 16 are formed between the respectiveseparation grooves 15 of the inner peripheral surface 13. Centers of theeccentric grooves 16 are eccentric with respect to an axial center P ofthe outer diameter of the bearing 1. The eccentric grooves 16 areinwardly biased toward one rotation direction of the shaft 2 shown by anarrow R. In this case, as shown in the drawings, the number of theseparation grooves 15 is 5, and the number of the eccentric grooves 16is 5. These numbers are desirably 3 to 6.

A small gap between the inner surfaces of the eccentric grooves 16 and aperipheral circumferential surface of the shaft 2 is wedge-shaped incross section so as to be gradually narrower and smaller in the rotationdirection of the shaft 2. In this case, the separation groove 15 has awidth corresponding to an angle ? of 8 to 20 degrees in thecircumferential direction having the axial center P as a center as shownin FIG. 3. The separation groove 15 has a maximum depth of about 0.10mm.

A bearing gap of radial side is formed between the inner peripheralsurface 13 of the bearing 1 and the peripheral surface of the shaft bodyof the shaft 2 inserted into the shaft hole 11. A bearing gap of thrustside is formed between the upper end surface 12 of the bearing 1 and theflange of the shaft 2. Lubricating oil is supplied into the bearinggaps. For example, the bearing gap of radial side has a width of about 1to 3 μm, and the bearing gap of thrust side has a width of about 5 to 10μm.

In the bearing 1, when the shaft 2 inserted into the shaft hole 11 isrotated in the arrow R direction as shown in FIGS. 2 and 3, thelubricating oil is exuded to the respective separation grooves 15 of theinner peripheral surface 13 and is held therein. The lubricating oilheld therein is efficiently moved by the shaft 2, and enters into thewedge-shaped small gap between the eccentric groove 16 and the shaft 2,so that an oil film is formed. The lubricating oil entering the smallgap flows to the narrower and smaller side thereof, and it thereby isunder high pressure due to the wedge effect, so that a high radial fluiddynamic pressure is generated. Portions under high pressure in the oilfilm are generated at equal intervals in the peripheral direction inaccordance with the eccentric grooves 16. As a result, the radial loadof the shaft 2 is supported in a well-balanced manner to have highstiffness.

On the other hand, the lubricating oil is exuded to the respectivespiral grooves 14 formed on the upper end surface 12 of the bearing 1,and is held therein. One portion of the lubricating oil held therein ismoved from the respective spiral grooves 14 by the rotation of the shaft2, so that an oil film thereof is formed between the upper end surface11 and the flange. The lubricating oil held in each spiral groove 14flows from the peripheral side of each spiral groove 14 to the innerperipheral side thereof, so that a thrust fluid dynamic pressure isgenerated and it is highest at an end portion on the inner peripheralside thereof. The flange receives the thrust fluid dynamic pressure, sothat the shaft 2 is floated by small amount. As a result, the thrustload of the shaft 2 is supported with high stiffness in a well-balancedmanner.

Next, a production method for the above bearing 1 of the embodimentaccording to the present invention will be explained. FIG. 4 is adiagram showing a process order of the production method.

-   1. Compacting of Raw Powder and Sintering

First, a raw powder of a metal powder is compacted, so that a greencompact, which has a near net shape corresponding to the bearing 1, isobtained. The green compact is provided in a sintering furnace and issintered therein, so that a sintering compact having a porosity of 8 to20 vol % is obtained as a material.

-   2. Size Control by Repressing and Forming of Fluid Dynamic Pressure    Groove

Next, the obtained sintered bearing is set in a die having apredetermined shape and it is repressed therein, so that an outerdiameter, an inner diameter, and axial direction length (height) of thesintered bearing are controlled with a required size precision. By usinga core having protrusions corresponding to the above spiral grooves 14,spiral grooves 14 are transferred and formed on one end surface (theabove upper end surface 12) of the sintered bearing which has thecontrolled size. By using a sizing core having protrusions correspondingto the above separation grooves 15 and the above eccentric grooves 16,separation grooves 15 and eccentric grooves 16 are transferred andformed on an inner peripheral surface 13 of the sintered bearing.

-   3. Infiltration of Resin

The sintered bearing which has the spiral grooves 14 formed on the otherend surface and the separation grooves 15 and the eccentric grooves 16formed on the inner peripheral surface 13 is immersed in a resinsolution in vacuum condition. Next, the sintered bearing is opened tothe air. A resin solution is infiltrated into pores of the sinteredbearing by pressure difference between vacuum and the air. An anaerobicadhesive which is mainly composed of polyglycol dimethacrylate isdesirably used as the resin for the infiltration. The resin is cured byheating after being infiltrated into the sintered bearing. Since theinfiltrated resin is adhered so as to cover the entire surface of thesintered bearing, the resin on the entire surface including the innerperipheral surface 13 is removed by water washing before the resin iscured.

-   4. Barreling

The sintered bearing, of which the pores are sealed by the resininfiltration, is subjected to magnetic barreling or electromagneticbarreling. Fine stainless pin having a diameter of about 0.5 mm isdesirably used as media for the barreling. Many media give impacts tothe surface of the sintered bearing by the magnetic barreling or theelectromagnetic barreling. As a result, the entire surface of thesintered bearing, which includes the end surface having the spiralgrooves 14 formed thereon and the inner peripheral surface 13 having theseparation grooves 15 and eccentric grooves 16 formed thereon, issubjected to barreling by the media. And the resin, which is adhered tothe surface of the sintered bearing and cannot be removed by the waterwashing, is completely removed, and the surface thereof becomes clean.

When the infiltrated resin is the anaerobic adhesive, the volume of theresin expands in the curing by the heating, and the resin is exuded tothe surface of the sintered bearing, so that the small amount of theresin may remain on the surface of the sintered bearing. When the poresare filled with the resin and the sintered bearing is cooled to roomtemperature after the curing, the store of the resin contracts, so thatthe small amount of the pores remains. Thus, when the resin remains onthe surface or, in contrast, the pores remains thereon, the surface ofthe sintered bearing is subjected to the barreling, so that theremaining resin is removed or the remaining pores are closed by plasticflow due to the impacts of media and completely sealed.

-   5. Resin Coating

The entire surface of the sintered bearing, which was subjected to thebarreling, is covered with a resin by the following coating method, sothat a resin coating layer is formed thereon. In the coating method, thesintered bearing is immersed in a resin solution for coating or a resinresolution for coating is sprayed onto the entire surface of thesintered bearing. A material of the resin may be acrylic one or epoxyone. The material of the resin is desirably composed of fluororesinwhich is quick-drying and is superior in oil repellency. In order not toinfluence on size precision of the sintered bearing, the coating layerhas a thickness of 5 μm or less, and desirably has a thickness of about1 μm.

In the production method of the fluid dynamic pressure sintered bearingof the embodiment according to the present invention, the size controlof the whole sintered bearing is performed by the repressing. Next, theformation of the spiral grooves 14 on the upper surface 12 and theformation of the separation grooves 15 and the eccentric grooves 16 onthe inner peripheral surface 13 are performed by the repressing. Afterthat, the sealing of the pores by the resin infiltration, the barreling(magnetic barreling or electromagnetic barreling), and the resin coatingare performed in this process order. As a result, the fluid dynamicpressure sintered bearing is obtained.

In this method, since the sealing of the pores is performed by the resininfiltration, in comparison with a case sealing is performed bymechanical impacting (for example, shot blasting), deformation of thespiral grooves 14, the separation grooves 15, the eccentric grooves 16,the inner peripheral surface 13 supporting the shaft 2, and the like canbe prevented, and decrease in the fluid dynamic pressure can beprevented by the sufficiently sealing of the pores. Since the resininfiltration is performed after the all repressing, adhesion of theresin to the male die for the repressing can be prevented, so thatdeterioration of size precision of the sintered bearing, which may becaused by the adhesion, can be prevented. Since in the repressing, thesintered bearing is simply composed of sintered compact having theunsealed pores, the spring back after the repressing of the sinteredbearing and the ejection of the sintered bearing from the die ismaintained to be small, and the transfer properties of the fluid dynamicpressure grooves onto the sintered bearing are maintained to be good.Since the sintered bearing is subjected to the barreling after the resininfiltration and the entire surface of the sintered bearing is polished,the resin remaining on the surface or the pores remaining thereon areremoved by the barreling, so that the size precision can be maintained.Due to these, in the obtained fluid dynamic pressure sintered bearing,the pores can be sufficiently sealed and the decrease of fluid dynamicpressure can be prevented. In addition, since the size precision can beimproved, bearing performances (for example, rotational performance ofhigh speed and high precision, and low noise) can be improved.

Since the entire surface of the sintered bearing is cleaned by thebarreling, the resin coating layer can be formed to be good. In thiskind of fluid dynamic pressure sintered bearing for motors, a Fe—Cubased metal material is often used therefor from a view point of thetime of initial running-in and the strength, but this material easilyrusts in the air. However, since the resin coating layer is formed onthe surface of the sintered bearing, the water repellent effects can beimproved, and the rusting can be effectively prevented.

1. A production method for a fluid dynamic pressure sintered bearing,comprising: preparing a sintered bearing having a porosity of 8 to 20vol % as a material; controlling at least one of an overall length, anouter diameter, and an inner diameter of the sintered bearing byrepressing the sintered bearing; forming grooves for generating a fluiddynamic pressure on a bearing surface of the sintered bearing byperforming repressing and plastic working on the sintered bearing;sealing pores exposed on the bearing surface by infiltrating a resininto at least the pores; and barreling entire surface of the sinteredbearing by magnetic barreling or electromagnetic barreling.
 2. Aproduction method for a fluid dynamic pressure sintered bearingaccording to claim 1, the production method further comprises: forming aresin coating layer on the entire surface of the sintered bearing afterthe barreling.
 3. A production method for a fluid dynamic pressuresintered bearing according to claim 2, the resin coating layer iscomposed of a fluororesin and has a thickness of 5 μm or less.